1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a plate-like workpiece such as a semiconductor wafer or a glass substrate.
2. Description of the related art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Although the photolithographic process can form interconnections that are at most 0.5 xcexcm wide, it requires that surfaces of semiconductor wafers on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small. It is, therefore, necessary to planarize the surfaces of the semiconductor wafers for photolithography. One customary way of planarizing the surfaces of the semiconductor wafers is to polish them with a polishing apparatus.
FIG. 13 of the accompanying drawings shows a main part of a conventional polishing apparatus. The polishing apparatus comprises a rotating polishing table 100 with a polishing cloth 102 made of urethane or the like attached to an upper surface of the polishing table 100, a top ring (workpiece holder) 104 for holding a semiconductor wafer W which is a workpiece to be polished and pressing the semiconductor wafer W against the polishing table 100 while the top ring 104 is rotated, and a polishing liquid supply nozzle 106 for supplying a polishing liquid Q to the polishing cloth 102. The top ring 104 is connected to a top ring shaft 110 through a spherical bearing 108 so that the top ring 104 is tiltable with respect to the top ring shaft 110. The top ring 104 is provided with an elastic pad 112 made of polyurethane or the like on its lower surface, and the semiconductor W is held by the top ring 104 in contact with the elastic pad 112. The top ring 104 also has a cylindrical guide ring 114 mounted on a lower outer circumferential edge thereof for retaining the semiconductor wafer W on the lower surface of the top ring 104.
In operation, the semiconductor wafer W is held against the lower surface of the elastic pad 112, and pressed against the polishing cloth 102 on the polishing table 100 by the top ring 104. The polishing table 100 and the top ring 104 are rotated to move the polishing cloth 102 and the semiconductor wafer W relative to each other. At this time, the polishing liquid Q is supplied onto the polishing cloth 102 from the polishing liquid supply nozzle 106. The polishing liquid Q comprises a chemical solution such as an alkali solution containing abrasive particles suspended therein. The semiconductor wafer W is polished by a composite action comprising a chemical polishing action of the chemical solution and a mechanical polishing action of the abrasive particles. This polishing is called chemical mechanical polishing.
In the chemical mechanical polishing (CMP) apparatus using the polishing cloth 102, since the polishing cloth 102 is made of material having elasticity, irregularities of a polished surface of the semiconductor wafer remain, and the surface of the semiconductor wafer cannot be sufficiently planarized. Therefore, the conventional CMP apparatus cannot cope with a demand for a higher degree of planarization of the semiconductor wafer.
To be more specific, a device pattern on the upper surface of the semiconductor wafer W has various irregularities having various dimensions and steps. When the semiconductor wafer W having step-like irregularities is planarized by the polishing cloth 102 having elasticity, not only raised regions but also depressed regions are formed. Hence, irregularities of the polished surface of the semiconductor wafer are difficult to eliminate, with the result that a high degree of flatness of the polished surface cannot be obtained.
Further, the surface of the polishing cloth 102 tends to have irregularities, and hence, it is necessary to frequently perform dressing of the surface of the polishing cloth 102 to remove glazing of the surface of the polishing cloth 102.
Furthermore, a considerable proportion of the polishing liquid Q supplied to the polishing cloth 102 is discharged without reaching the surface of the semiconductor wafer to be polished. Consequently, the polishing liquid Q is required to be supplied in a large quantity, and hence, an operating cost in the polishing process becomes high because the polishing liquid is expensive and the cost of a process for treating the polishing liquid is high.
Therefore, there has been developed a fixed abrasive type of polishing apparatus and method in which a polishing surface comprising an abrading plate, i.e., a fixed abrasive plate is used, in place of the polishing cloth 102. The abrading plate comprises abrasive particles such as silica particles and a binder for binding the abrasive particles, and is formed into a flat plate. FIG. 14 shows a main part of a conventional polishing apparatus having such abrading plate. The polishing apparatus comprises a polishing table 100 with a polishing tool 120 attached to an upper surface thereof, and liquid supply nozzles 124 connected to a liquid supply device 122 for supplying water or a chemical liquids during polishing. The polishing tool 120, attached to the upper surface of the polishing table 100, comprises a base plate 116 and an abrading plate 118 attached to the surface of the base plate 116. Other structures of the polishing apparatus shown in FIG. 14 are the same as that of the conventional polishing apparatus shown in FIG. 13.
According to the above polishing process, the abrading plate (fixed abrasive) is harder than the polishing cloth and has less elastic deformation than the polishing cloth. Hence, only the raised regions on the semiconductor wafer are polished and undulation of the polished surface of the semiconductor wafer is prevented from being formed. Therefore, selective polishing performance of the raised regions on the semiconductor wafer is improved, a degree of flatness of the semiconductor wafer is improved, and an expensive polishing liquid Q is not required to be used.
Further, it is confirmed by the inventors of the present application that in the polishing method using the fixed abrasive, the polished surface of the semiconductor wafer is planarized once to a certain level, and then the polishing rate is lowered extremely to show a self-stop ability of polishing because of nature of the fixed abrasive. Therefore, the inventors of the present application have proposed to utilize such self-stop ability of polishing for detecting an endpoint of polishing or detecting a thickness of a film formed on the semiconductor wafer W in Japanese Patent Application Nos. 10-150546 and 10-134432.
Recently, there have been strong demands in the polishing apparatus for polishing semiconductor wafers field for an improvement of productivity per apparatus and improvement of productivity per unit installation area of the apparatus, as in other semiconductor manufacturing apparatuses. However, in the polishing apparatus having a single top ring per polishing table, the polishing surface on the polishing table is not effectively utilized, and therefore, the productivity per unit installation area of the apparatus cannot be improved.
In order to solve the above problem, it is conceivable that a plurality of top rings are provided with respect to a single polishing table for thereby utilizing a polishing surface on the polishing table efficiently. However, in this case, the polishing surface on the polishing table is rapidly deteriorated and the polishing rate is lowered, and frequently conducted dressing operations lower an operating efficiency of the polishing apparatus. Particularly, in a case of the fixed abrasive method, it is necessary to dress the polishing surface on the polishing table frequently to regenerate and planarize the polishing surface because the polishing surface is worn away by the polishing operation and irregularities of the polishing surface are formed.
Further, in a case of conducting finish polishing of the semiconductor wafer, in order to avoid formation of fine scratches on the polished surface of the semiconductor wafer, it is necessary to use a fixed abrasive having different compositions or a different polishing table having a polishing cloth thereon. Hence, throughput of the semiconductor wafers is greatly lowered.
It is therefore an object of the present invention to provide a polishing apparatus which has a high processing capability per unit time and unit installation area in a clean room requiring an expensive operating cost.
According to an aspect of the present invention, there is provided a polishing apparatus for polishing a surface of a workpiece. The polishing apparatus comprises a polishing table having a polishing surface thereon, a plurality of workpiece holders each for holding a workpiece and pressing the workpiece against the polishing surface, and a dresser for dressing the polishing surface by pressing a desired position of the polishing surface.
According to the present invention, when workpieces, such as semiconductor wafers, are polished by utilizing a polishing surface efficiently, even if a deteriorating rate of the polishing surface is high, the workpieces can be efficiently polished to a high degree of flatness while forming a good polishing surface at all times and regenerating the polishing surface by dressing.
In a preferred aspect of the present invention, the polishing surface has a plurality of polishing positions which have different dressing effects.
In the case where the polishing table is a rotating-type polishing table, the polishing surface has the most efficient polishing performance at a polishing position immediately downstream of the dressing position, in a rotational direction of the polishing table, where dressing of the polishing surface is performed. The polishing surface has less efficient polishing performance at other polishing positions where deterioration of the polishing surface progresses because at least one of the workpieces has been polished once. By utilizing these characteristics, the workpieces can be polished under different polishing conditions at different polishing positions.
In a preferred aspect of the present invention, the workpiece is sequentially polished by moving the workpiece to the plurality of polishing positions sequentially.
In a preferred aspect of the present invention, an initial polishing of the workpiece is conducted at the polishing position where the dressing effect remains large, and a secondary polishing or a finish polish of the workpiece is conducted at the polishing position where the dressing effect remains small.
In a preferred aspect of the present invention, the polishing pressure applied to the workpiece by the workpiece holder is controlled on the basis of the dressing effect remaining on the polishing surface. If the dressing effect remains large at a certain polishing position, the polishing pressure applied to the workpiece and/or the relative sliding speed between the workpiece and the polishing surface are decreased. Conversely, if the dressing effect remains small at a certain polishing position, the polishing pressure applied to the workpiece and/or the relative sliding speed between the workpiece and the polishing surface are increased.
In a preferred aspect of the present invention, the polishing surface has a plurality of polishing positions, and dressing effects on the plurality of polishing positions by the dresser are equal to one another.
With this arrangement, a plurality of polishing positions on the polishing surface where a plurality of workpiece holders are located can be kept at a constant polishing performance having a certain level. Hence, a plurality of workpieces can be polished under the same polishing condition.
In a preferred aspect of the present invention, a plurality of dressers are provided so as to correspond to the plurality of workpiece holders.
In a preferred aspect of the present invention, the dresser is provided so as to dress an entire surface of the polishing surface.
A dressing load applied to the polishing surface by the dresser or the relative speed between the dresser and the polishing surface may be controlled depending on the number of workpieces which are polished simultaneously. For example, if the number of workpieces is large, the dressing load is increased, and if the number of workpieces is small, the dressing load is decreased. Thus, the dressing load is controlled according to the degree of deterioration caused by polishing operation.
In a preferred aspect of the present invention, the polishing surface comprises a polishing cloth, or a fixed abrasive plate having a self-generation function of abrasive particles.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.